The present invention relates to a surround reproducing circuit for inputting a stereophonic signal for two channels of an L signal (a left signal) and an R signal (a right signal), thereby producing a surround effect by means of two speakers.
Most of a frequency distribution of a voice which can be heard by a human concentrates in the vicinity of 300 Hz to 3.5 KHz. A frequency of 1 KHz is important to the articulation of a conversation and a wavelength thereof is approximately 30 cm. Accordingly, if the voice arrives from the left in a transverse direction of a head, it reaches a right ear in an opposite phase to a left ear because the right ear is distant by approximately 15 cm as compared with the left ear. More specifically, in the case in which the same sound arrives from the left and the right, a listener feels that an image of sound source is present on the front.
Referring to the L-R signal, however, a change in a phase and a difference between sound volumes are made for the R signal. Therefore, the source of sound of the L-R signal is localized on the left side within a range of 180 degrees so that the human feels that the sound comes from just the left side. Referring to the R-L signal, similarly, the source of sound of the R-L signal is localized on the right side within a range of 180 degrees so that the human feels that the sound comes from just the right side.
On the other hand, in the case in which a sound comes from the front, a voice frequency band (300 Hz to 5 KHz) is emphasized through an earlobe and an ear hole. In the case in which the sound comes from just the side, a frequency characteristic is almost flat.
In order to cause the sound of the L-R signal coming from the front to pretend to be a sound coming from the left side, accordingly, it is necessary to reduce the level of a voice frequency band (300 Hz to 5 KHz) of the L-R signal by a predetermined amount. In order to cause the sound of R-L coming from the front to pretend to be a sound coming from the right side, similarly, it is necessary to reduce the level of the voice frequency band (300 Hz to 5 KHz) of the R-L signal by a predetermined amount.
A conventional surround reproducing circuit having one speaker arranged in each of left and right front portions generates an L side surround signal to be an L-R signal from an L signal and an R signal of a stereo which are inputted from input terminals 21 and 22 by means of an adding circuit 23, and inputs a difference signal to a band-elimination filter 24 having a frequency characteristic of FIG. 9, thereby reducing the level of a voice frequency band (300 Hz to 5 KHz) as shown in FIG. 8.
The gain of the L side surround signal having a frequency characteristic thus regulated is further regulated by means of a gain variable amplifier comprising an operational amplifier 25 and resistors R7 and R8, and is exactly added to an L signal line by means of an adder 26 and is converted into an R side surround signal by phase inversion by means of an adder 27 to be added to an R signal line, and is thus outputted to output terminals 28 and 29. The reference numerals 30, 31, 32 and 33 denote a buffer.
Thus, a voice signal component which is easy to understand a sense of direction emphasized by a human ear is removed and a reverberation sound or an echo sound in a frequency band which is hard to understand the sense of direction is intensified and mixed with the L signal or the R signal to emphasize a change in a phase and a difference between sound volumes. Thus, a surround effect is realized.
FIG. 10 is a diagram showing another conventional surround reproducing circuit, in which a higher order band-elimination filter is constituted by an operational amplifier 34, a parallel circuit of a resistor R9 and a capacitor C3 connected between an output terminal of the operational amplifier 34 and an inversion input terminal, and a series circuit of a resistor R10 and a capacitor C4 connected between the inversion input terminal and a ground. By the band-elimination filter, the level of a voice frequency band (300 Hz to 5 KHz) is reduced in the same manner as in the band-elimination filter 24 in FIG. 8.
In a circuit for using a plurality of filters higher than second order to enhance a surround effect, there is a problem that a change in a phase is increased to make the localization of an image of sound source unclear and to cause a surround having a sense of distortion.
In the conventional surround reproducing circuit shown in FIG. 8, furthermore, at least two capacitors are required for constituting the band-elimination filter 24 and so is a surround reproducing circuit shown in FIG. 10. These capacitors require a generally large capacitance. It is hard to constitute the capacitors in an IC when forming the whole as IC. For this reason, it is necessary to externally attach the capacitors. Therefore, there is a problem that the number of IC pins is increased.
In the conventional surround reproducing circuit shown in FIGS. 8 and 10, furthermore, if an interval between speakers is small, for example, 20 cm or less, the surround signals added in opposite phases to each other through the adders 26 and 27 are offset in a space. Consequently, there is also a problem that a sufficient surround effect cannot be obtained.
It is an object of the present invention to provide a surround reproducing circuit in which a change in a phase is not increased and the localization of a source of sound becomes definite, and furthermore, a filter can be simplified and an excellent surround effect can be obtained even if an interval between speakers is small.
In accordance with a first aspect of the present invention, there is provided a surround reproducing circuit which includes a first adder for generating a difference signal of an L signal and an R signal which are inputted, a low-pass filter connected to an output side of the first adder, and second and third adders for mixing an output signal of the low-pass filter as a surround signal with the L signal and the R signal in an opposite phase relationship to each other.
In accordance with a second aspect of the present invention, a phase-shifting circuit having an almost constant gain in a full frequency band of the input signal and serving to carry out a phase shift for a change from 0 to 180 degrees according to an increase in a frequency of the input signal is connected to an output side of the second or third adder.
In accordance with a third aspect of the present invention, an amplifier or an attenuator is connected to the output side of the second or third adder and a gain difference between a channel of the L signal and a channel of the R signal is set to be 3 dB or more.
In accordance with a fourth aspect of the present invention, the low-pass filter has a cut-off frequency of 700 Hz to 2 KHz and an attenuation characteristic of xe2x88x926 dB/oct.
In accordance with a fifth aspect of the present invention, a gain variable amplifier is inserted into an output side of the low-pass filter and an output signal of the gain variable amplifier is inputted to the second and third adders in an opposite phase relationship to each other.
In accordance with a sixth aspect of the present invention, the phase-shifting circuit is replaced with a phase-shifting circuit having an almost constant gain within a full-frequency band of the input signal and serving to carry out a phase shift for a change from 90 to 175 degrees within a frequency band of 300 Hz to 3.5 KHz.
In accordance with a seventh aspect of the present invention, in which the phase-shifting circuit is replaced with a phase-shifting circuit having an almost constant gain within a full-frequency band of the input signal and serving to carry out a phase shift from 120 to 170 degrees at a frequency of 1 KHz.